The present invention relates to coherent imaging systems using vibratory energy, such as ultrasonic or electromagnetic waves, and more particularly to a novel method and novel apparatus for providing dynamic focussing of the coherent imaging beam during reception.
It is well known that the formation of a vibratory energy beam in coherent imaging systems, such as ultrasound or electromagnetic imaging systems and the like, forms a reception beam by utilization of precision time delay of the signals received by each transducer channel of a transducer array. This beamforming process is generally separated into two parts: the steering function and the focussing function. The steering function uses a first portion of the beamforming time delay to provide that time delay necessary to steer the beam in a given direction .theta., with respect to a line normal to the face-plane of the array. The second time delay, which is itself time dependent, is the time delay necessary to maintain accurate focus during travel of the imaging energy pulse through the sample (e.g. the human body being imaged). While it is well known to provide method and apparatus for fully digital beam formation in a phased array coherent imaging system, such as described and claimed in U.S. Pat. No. 4,839,652, issued Jun. 13, 1989, assigned to the assignee of the present invention and incorporated herein in its entirety by reference, it is highly desirable to provide simplified apparatus and methods for computing and generating the time delay variations which are necessary for dynamic focus in such a system.